In recent electronic instruments, a demand of a high-speed signal treatment and digitization for imparting high function have been increased more or more. A laminate comprising a nonwoven fabric of an aromatic polyamide (hereinafter referred to as an "aramid", sometimes) as a substrate has characteristics such as low dielectric constant, lightweight and low linear thermal expansion coefficient and it's application for such a field has been developed.
For example, U.S. Pat. No. 5,314,742 discloses that a nonwoven fabric comprising fibrils of a meta-oriented aromatic polyamide (hereinafter, referred to as a "meta-aramid", sometimes) and a para-oriented aromatic polyamide (hereinafter, referred to as a "para-aramid", sometimes) flock is useful as a substrate of a laminate having a low linear thermal expansion coefficient. In addition, Japanese Patent Kokai (Laid-Open) No. 5-327148 discloses that a linear thermal expansion coefficient in the planer direction can be reduced by using a cloth, paper or nonwoven fabric containing not less than 50% of para-aramid fibers as a substrate. However, the nonwoven fabric had a drawback that an uniform product can not be easily obtained as characteristics of the process.
In a board, those using a thermosetting resin such as a glass epoxy substrate have been exclusively used as a matrix resin together with a paper or nonwoven fabric, heretofore, and it is suggested to use the thermoplastic resin as the matrix resin as described in Japanese Patent Kokai (Laid-Open) No. 6-252555. The gazette discloses the improved structure when using a composite material prepared by impregnating a core material of the fiber fabric being with a thermoplastic resin as a multi-layer laminate. A matrix resin prepared by formulating a thermoplastic resin and a thermosetting resin in a predetermined proportion is also described, for example, in Japanese Patent Kokai (Laid-Open) No 57-165451 and U.S. Pat. No. 3,530,087. The former discloses a composition of a cyanate resin composition and polyether sulfon and the latter discloses a composition of an epoxy resin and polyether sulfon.
On the other hand, as the process of such a laminate, a novel process of producing a circuit substrate without using a conventional laminate has been put to practical use with recent miniaturization/lightweight of electronic instruments. For example, Japanese Patent Kokai (Laid-Open) No 7-147464 discloses a coefficient process of producing a printed circuit board of a high-density from a prepreg prepared by impregnating a nonwoven fabric of heat-resistant synthetic fibers with a thermosetting resin in the uncured state. If a complicated process which is carried out in a laminate manufacturer becomes unnecessary with the innovation of such a production procedure, a large rationalization can be expected. Therefore, the appearance of a lightweight prepreg having uniform formation, low linear thermal expansion coefficient and good mechanical strength is expected.